In the past, broadband communication was achieved by providing high bandwidth cable connections between a provider and a client. The cables are commonly in the form of coaxial cables, twisted pair copper telephone lines, or fibre optic cables. A broadband signal is transmitted down the cable toward the client end. For example, cable television involves a same signal transmitted to each client within a group of clients. The signals are sent in one direction only thus providing a one directional data link, Each client then decodes portions of the signal as desired.
Information being sent to the clients, commonly called the down link or forward link, is usually transmitted at a much higher capacity than the up link or return link. Such asymmetry in communications prevents clients from both generating and distributing substantial amounts of information into the network. As a consequence, the distribution of high content information by the network is exceedingly hierarchical, requiring the producers of information to access the hub nodes wherefrom the information is transmitted to the clients. Generally, subscribers desire a high bandwidth feed from a service provider and lower bandwidth feed to the service provider. An arrangement is similar for television on demand where television programs are provided to a client from the service provider and clients only send small data packets including ordering information etc. to the service provider.
It is well known that one of the most expensive aspects of any broadband communication network is the cost of running cable from a service provider to each client. For example, fibre optic cables are estimated to cost tens of thousands of dollars per mile of installed cable This results from labour costs, costs of routing the cables and obtaining rights to bury the cables, costs of repairing damaged cables and so forth. It would be highly advantageous to obviate any need to lay new cable in order to support broadband communication over a wide area and more particularly bidirectional communication between a service provider and a client subscriber or between client subscribers, especially in view of the growing need by clients to be able to access and use ever increasing amounts of return link capacity. Additionally, adding the capability to existing wireline infrastructure is expensive and problematic, often requiring the special conditioning of old lines or the addition of new infrastructure. In many areas, infrastructure is simply not in place, forcing those desiring broadband communications to face the daunting task of not only installing links to clients, but also installing totally new switching systems.
The convergence of the Internet and television services has given rise to a need to provide high bandwidth bi-directional communication. Other applications such as video conferencing, require high bandwidth in both transmit and receive directions. Conventional broadband data distribution services are not capable of supporting such requirements.
In an attempt to overcome this problem, cable service providers have released a cable modem for use in providing Internet services over a standard cable connection. Such a connection has many known problems. The bandwidth is limited by the cable itself and division of subscribers into groups requires the addition of new hardware. The bandwidth to any given client is limited by the number of active subscribers in their group and by the physical limitations on the information carrying capacity of the coaxial cable. Further, such a system is highly asymmetric and incapable of, for example, supporting extensive video interactivity between client subscribers.
Telephone system operators have also tried to overcome the above noted problems with broadband data distribution by developing high speed modems that condition signals for distribution over twisted pair telephone lines. These modems, called Digital Subscriber Line (DSL) modems, are most commonly represented by a class of modems called Asymmetric DSL (ADSL). Some of the drawbacks of these systems are that they require the twisted pair telephone lines to be in good condition in order that the modems can be effectively used.
Also, networks employing ADSL are highly asymmetrical, having forward link capacity significantly larger than return link capacity. In addition, unless the ADSL links are in good condition and relatively short, supporting high quality video services becomes problematic. Another significant problem faced with ADSL is the requirement for a public telephone subscriber line network to be in place in order to facilitate deployment. Such a requirement for an in-place infrastructure is simply impossible in many regions of the world, hence deployment of ADSL is limited to those regions that are well developed economically and have a solid technological base.
Another attempt to overcome the above known problems is known Local Mutlipoint Distribution Service (LMDS) or Local Multipoint Communication Systems (LMCS). LMDS (LMCS) is a wireless, two-way broadband technology designed to allow network integrators and communication service providers to bring a wide range of high-value, quality services to homes and businesses. Services using LMDS technology include high-speed Internet access, real-time multimedia file transfer, remote access to corporate local area networks, interactive video, video-on-demand, video conferencing, and telephony among other potential applications.
In the United States, the FCC became interested in the possibility of LMDS bringing long needed competition to the telecommunication marketplace, where it has the use of 1.3 GHz of RF spectrum to transmit voice, video and fast data to and from homes and businesses. With current LMDS technology, this roughly translates to a 1 Gbps digital data pipeline. Canada already has 3 GHz of spectrum set aside for LMCS. Many other developing countries see this technology as a way to bypass the expensive implementation of cable or fiber optic networks.
LMDS uses low powered, high frequency (25–31 GHz) signals transmitted over a distance of 3–5 kilometers. These zones of coverage, or cells, are created by sectorial antennas and switching systems mounted on rooftops of urban buildings and towers. These cells are typically spaced 4–5 kilometers (2.5–3.1 miles) apart. LMDS cell layout determines the cost of building transmitters and the number of households covered. With circular cells of 4 kilometers in radius, about 50 square kilometers falls within a single cell. In urban areas, this translates to about 80,000 homes within a single cell.
Despite the promise of LMDS there are a number of issues that compromise its acceptance as a widely deployed and ubiquitous wireless multimedia distribution system. One significant drawback to the deployment of LMDS systems is the large signal attenuation that is faced by the frequencies used in transporting the data between the hub and subscriber terminals. This signal attenuation is exacerbated by rain, foliage, building blockage, and other factors related to the absorption, refraction, or reflection of the signal by obstacles in the propagation path. The underlying phenomena are well understood but are of such severity that current deployments of LMDS are limited to situations where the links are free from obstructions and where attenuation by rain or snow is sufficiently countered.
Another issue, also related to propagation characteristics, is that the intended polarization reuse schemes for increasing the capacity of LMDS systems are also prone to degradation by the same phenomena that attenuate the signal. As a consequence there are issues of deployment and capacity, which bring into question the economic viability of LMDS systems.
Though there is much promise in LMDS systems operating at 28 GHz, the effect of the propagation environment makes wide scale deployment problematic. Though initial expectations assumed this technology could provide services to everybody within a coverage area, most current plans are for providing service only to those locations where a signal is received without even nominal propagation impairments. Also, current trends in LMDS are mainly for providing commercial access to broadband communication.
In view of the problems faced by LMDS due to its use of frequencies around a 28 GHz band, the most obvious solution to the above noted problems is to develop a system using frequencies more robust to propagation degradation. Typically these frequencies are in bands substantially lower than 28 GHz. The difficulty with using bands at lower frequencies is that there is insufficient bandwidth to support the density of communications demanded by broadband digital services and what bandwidth is available is often shared with other users such as satellite systems or dedicated point to point communications links. In order to effectively use other frequency bands for broadband wireless communications it is therefore necessary to devise a system which can co-exist with primary users and in addition, use limited bandwidth in such a manner that effective broadband communications can be established. At present, such a solution is unavailable for use with the types of data throughput anticipated and currently required.
A great deal of the early discussion of LMDS applications centered on the transmission of video. With the recent surge of interest in the Internet and the accompanying demand for bandwidth, fast data appears to be the greatest application for this technology. With 1.3 GHz of spectrum, LMDS can provide a pipeline for a great deal of data. Homeowners pay about $30 per month for video, but businesses regularly pay over $1000/month for a high speed T1 (1.544 Mbps) line from phone companies.
Using only the 850 MHz unrestricted bandwidth, along with a modulation scheme such as quadrature phased shift keying (QPSK), well over 100 T1 equivalent lines can be provided in a cell formed by an omnidirectional LMDS transceiver even without splitting cells into separate sectors. Though it has been proposed that by using horizontal and vertical polarized sectors within a cell, LMDS providers can re-use bandwidth and multiply the number of T1 equivalents available, insufficient polarisation isolation makes such claims dubious especially in applications wherein subscribers are individuals and not businesses. A typical commercial LMDS application is believed to be able to provide a downlink throughput of 51.84–155.52 Mbp/s and a return link of 1.544 Mbp/s (T1). This capacity translates into potential to provide “full service network” packages of integrated voice, video and high-speed data services. Actual service carrying capacity depends on how much of the spectrum is allocated to video versus voice and data applications. Assuming that 1 GHz of spectrum is available, an all-video system could provide in the order of 275 channels of digital broadcast quality television plus on-demand video services. Unfortunately, LMDS has many known drawbacks and even though an LMDS television system was installed in Brooklyn in the early 1990's, it has failed to find wider acceptance.
It is an object of the present invention to provide a robust broadband wireless communication system for use in delivering data from a service provider to a plurality of clients. It is another object of the present invention to provide a system for operation at lower frequencies than LMDS, which provides many of the benefits of LMDS without many of the drawbacks.